Method and means for measuring altitude of aircraft



L. ESPENSCHIED METHOD AND MEANS FOR MEASURING ALTITUDE OF AIRCRAFTOriginal Filed April 29, 1930 Stream?,

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INVENTOR I Psoe/@schied ATTORN EY Patented June 23,

' UNITED STATES ,PATENT OFFICE METHOD Aun MEANs Foa MEASURING ALTITUDEor AIRCRAFT Lloyd Espenscliied, Kew Gardens, N. Y., assignor to AmericanTelephone and Telegraph Com pany. a corporation of New York riginalapplication April 29, 1930, Serial No. 448,409. Divided and thisapplication January l30, 1936, Serial No. 61,802

5 Claims.

surface, receiving Lthe Waves reected by the earths surface upward tothe craft and determining the time interval between radiation andreception. The distance of the craft above the earths surface ismeasured in terms of the time taken by the waves to make the completecircuit from the craft to the earth and baci: again. My inventionresides in a method and means for determining the 'time taken by thetransmitted and reflected waves to complete the circuit between thecraft and the earth and back again; which time is determined by the timedelay of the returned waves as compared with the outgoing waves. Thattime delay is measured by beating the received, reected waves with thewaves generated for transmission, whereby a resultant current is set upin an indicating device, which resultant rises and falls in magnitude ata frequency depending upon the time delay between the waves that arebeing beaten together.

My invention resides in a method and means by which the altitude ci thecraft above a reflecting surface may be determined by the time of travelof the electromagnetic waves from the craft to the surface and backagain to the craft.

This invention will be fully understood from the following descriptionwhen read in connection with the attached drawing of which Figure 1-shows schematically one `form of embodiment of the invention; Fig. la isa graph illustrating the description of the methods underlying myinvention; Fig. 2 shows schematically another arrangement of theapparatus differing from that in Fig. 1 in that the transmittedfrequency does not remain xed but varies continuously throughout apredetermined range; and Fig.' 3 illustrates a system combining thedirect indicating feature of Fig. 2 .with the use oi'separatetransmitting and receiving antennael as in Fig. l.

In Fig. l, l represents a source of high frequency oscillations, forexample, 30 megacycles,

which source may be any well known type of oscillator having meanswhereby its frequency may be adjusted at will over a range-of about tenper cent. of its normal value. Thel oscillator l is coupled by thecoupling transformer -3 with the transmitting antenna 2 illustrated asla Hertz doublet. The oscillations transmitted by the antenna 2 downwardover the path 4 will be reflected by the earths surface, and thereflected waves, indicated by the paths 5 and 6 will be received by thereceiving antenna 'l represented by the divided loop which is balancedwith respeci; to the transmitting antenna. The loop is connected withthe primary winding of the transformer t, thevsecondary of which isconnected across the input circuit of the push-pull detec- 31 tor. Thedetector is of the well known type comprising the balanced tubes Q andrle having their grids connected with the secondaries of the transformer8 and having their plates connected with a common output circuit thatincludes a source ected waves that are impressed by the transformertunen the detector. All of the apparatus shown in the figure would belocated upon the aircraft.

The manner in which the aforedescribed apparatus operates to determinethe altitude of aircraft is as follows: The oscillator l is started inoperation'and waves are projected by the transmitting antenna i downwardto the earths surface where they will be reiiected, then later receivedby the loop 'l and impressed upon the input 40 oi the push-pulldetector. Simultaneously, a portion of -the generated oscillations willbe fed to the detector directly over the circuit is. `'ll'i'ie phaserelation in the detector between the reflected wave current and thecurrent taken direct- 4 1y from the local source at any moment dpendsupon the exact distance between the craft and the earths surface. Asthis distance changes upwards, the phase relation between the twocurrents in the detector changes cyclically and the indication of themeter l2 alternately increases and decreases. Likewise, as the distancebetween the craft and the earth changes downwardly,V the phase relationbetween the current varies simi- 5 fere'nt altitudes:

larly. That relationship is graphically in nitude of the frequenciescorresponding to dii'- Fig. 1a wherein the ordinates represent themagnitude of the current, that is shown by the indieating device i2, andthe abscissae represent frequency. It will be seen from Fig. 1a that.for any fixed distance between the craft and the earth,

the variation of the frequency of the transmitted and received waveswill cause an increase and decrease of the resultant current shown uponthe device I2, the values of which, when plotted, as in Fig. la, showa'low frequency wave.

In making the measurement, the oscillator I is adjusted as to frequencyIuntil the indicator I2 shows a minimum value. The frequency of theoscillator thatv produces such minimum value is then noted. Let it beassumed that that frequency is f1 of Fig. 1a. Then without changing thealtitude of the plane, the frequency of the oscillator I is changeduntil the indicator, I2 goes through one cycle of reading and comes backto the next low point adjacent to the low point corresponding to thefrequency f1. That low point could be either the one corresponding to foor fz.

' Let it be assumed that the changed frequency is fa. It will thereforebe apparent that we have shortened the length of the transmitted wave tosuch extent that the new wave will travel over the same path and whenreceived will have substantially the same phase relation to itstransmitted wave as the original wave of frequency ,1i

bore to its transmitted wave. 'I'hat is to say, in

' the distance from the craft to the earth and back again to the craft,there willbe a definite number of oscillations of frequency ,f1 and,similarly,

a definite number of oscillations of frequency fr. This probably willVbe better understood if we assume certain numerical values to illustratethe operation. If the first frequency setting were an even 30 megacycles(10 meters) and the second setting were 30.3 megacycles'(9.9 meters), itis known that it took a change of one part inrone hundred to shift byone wave length the number of wave lengths contained in the path oftransmission from the'craft to earth and back again. The length of thepath would accordingly be one hundred wave lengths and since each waveis 10 meters, the total path would be 1000 meters. The distance ofthecraft above the earth would be one-half of that value, that is, 500meters. If, in another case, the interval between two consecutiveminimum points of the curve of Fig. la corresponds to a 5% change in theoriginal frequency, that would be equivalent to a change of one wavelength in twenty. if the wave. length were meters, the total length ofthe path traversed by `the wave would be 10x20, that is, 200 meters andthe actual distance from the craft to the ground would be one-half ofthat value, that 'is E00 meters. Hence, the following relationshipobtains:

wave length o Height: above earth: transmitting -zor, abbreviating:

' E 1.5 X 10l -Where Af=the change in frequency.

'Ib cllo'wing table gives an idea of the mag- The'. foregoing formulaand values of thev "frequency interval of the beats, as shown in thetable, are directly applicable .to the arrangements shown in Figs. 2 and3, hereinafter fully described. When a push-pull detector is employed ofthe type shown in Fig. 1, in which the indicator is connected in thecommon conductor of the two tubes, the formula and the frequencyinterval of the table must be divided by two in order to represent thetrue altitude and the frequency interval, the reason being that with theindicator thus connected the beats occur twice as frequently as rtheywould with the type of detector shown in Figs. 2 and 3. and also twiceas frequently as they would if the indicator of Fig. 1 were bridgedacross the Aresistance in the output circuit of the detector of Fig. 1.From this table it is seen that the use of very high frequency currentsis required in order that the shorter elevations may be detected.

The arrangement shown in Fig. 2 is adapted to give a continuousindication of the distance of the craft above the earth and in that wayitv diilers from the arrangement shown in Fig. 1. In Fig. 2, the tubefunctions as a combined oscillatorand detector, its input and outputcircuit beingcoupled by the inductance 2|, which inductance forms partof the tuned circuit. That circuit also includes a fixed condenser '22and a variable condenser 23 which, by virtue of its connection with theshaft of the motor 24 is capable of varying continuously the amount ofcapacity in the tuned circuit. The motor may be driven by a generator orother source of potential that may be connected with the motor by thelead 25. The tuned circuit is connected with' the antenna 26 that is ofthe same type as the transmittingv r antenna of Fig. l. 'Ihe outputcircuit of the tube 20 is connected by the-transformer 21 with anindicating circuit that includes a frequency meter 28 and one or morestages 29 of alternating current amplicatlon. All of the apparatus shownin the figure would be located in suitable position on the aircraft.

In the operation of the arrangement shown in Fig. 2, the waves ofcontinuously varying frequency are radiated by the antenna 26, whichwaves, after being reiiected by the earth are again received by the saidantenna 26. The reflected waves are impressed upon theoscillator-detector 20 and the resultant wave, when amplified by 28,will be impressed upon the frequency meter 28. The amplitude of theresultant current at any instant will, ofcourse, depend upon theinstantaneous phase relation existing at that time between the currentresultingfrom the received wave, viz., the reflected wave, and thecurrent corresponding to the oscillations being produced in theoscillator-detector itself. Since the phase relation is continuouslychanging cyclically, the resultant current will undergo a cyclic changeof amplitude thus producing an alternating current of low frequency. Theinvention will be underwaves after being reflected from the earth,beating the received waves together with the generated Waves andadjusting the phase relationship of the beaten waves to produce aresultant current of minimum value, thenl changing the frequency of thegenerated waves, and, in lilke manner, radiating and receiving thereected waves of changed frequency, the degree of change being such asto cause the resultant current of the beating operation to rise to amaximum then fall to the next minimum value, and noting the diiferencebetween the frequencies for the two minimum values of resultant current.

3. In a system for determining the distance between two bodies bytransmitting waves from one body and receiving those waves afterreection from the other body, the combination upon one of said bodies ofa. source of radio Waves, having means for cyclically changing thefrequency thereof, of an antenna for transmitting the said Waves, a loopantenna adapted to be orientated to receive directly from thetransmitting antenna a controllable amount of the transmitted waves, andalso the wave refiected Yby said other body, means connected to saidloop antenna to beat together the reflected waves and the said directlyreceived waves, and an indicating device responsive to the resultantcurrent.

4. In a system for determining the altitude oi' an aircraft above areflecting surface, the combination upon said aircraft oi a source ofradio frequency oscillations having means to `vary the frequency thereofperiodically throughout a given range, a given `number of times persecond, an antenna to transmit those oscillations, a loop antenna ofnxed tuning adapted to receive oscillations reflected from the saidsurface and also oscillations directly from said transmitting an- 1tenna, the amount of power received directly from the transmittingantenna being controllable at will, means connected to the receivingantenna to beat together the reflected oscillations and those receiveddirectly from the transmitting an- 11 tenna, and means responsive to abeat frequency to indicate the altitude of the craft in terms ofdistance.

5. In a system for determining the altitude of an aircraft above areiiecting surface, the combil: nation upon said aircraft of a generatorof radio frequency waves having means for varying the frequencythereof,lan antenna connected to the said source to transmit the saidwaves, another antenna, the tuning of which remains iixed in 2 itsfrequency characteristic, for receiving waves reiiected from the saidsurface and also a controllable amount oi the waves transmitted from themst-mentioned antenna, the said antennae being capable of being balancedagainst each 2i! other but normally being unbalanced to such degree thatthe said other antenna will receive the desired amount of the directwaves from the transmitting antenna, means for beating together the saidreceived waves, and means responsive to the 38 beat frequency toindicate the altitude of the craft in terms of distance.

LLOYD ESPENSC.

